Effective theory for the two-nucleon system

We apply the method of unitary transformations to a model two-nucleon poten tial and construct from it an effective potential in a subspace of momenta below a given cut-off Lambda. The S-matrices in the full space and in the s ubspace are shown to be identical. We solve numerically the Schrodinger equ ation in the small momentum space and recover exactly the bound and scatter ing states of the full theory. We then expand the heavy repulsive meson exc hange of the effective potential in a series of local contact terms and dis cuss the question of naturalness of the corresponding coupling constants, U sing our exact effective theory we address further issues related to the ch iral perturbation theory approach of the two-nucleon system. The coordinate space representation of the effective potential is also considered. (C) 19 99 Elsevier Science B.V.